1. Field of the Invention
The present invention relates to a load cell in which at least the load-sensing member (deflecting part) of the bending beam is made of a ceramic material.
2. Description of the Prior Art
A load cell of this type is disclosed in Japanese Patent Laid-open No. 273029/1988. It is made up of a ceramic bending beam and strain gages of thin-film resistance formed thereon. Owing to its ceramic bending beam, this load cell has an advantage that it is immune against plastic deformation, very little sensitive to temperature, and highly resistant to any environment. Another advantage of this load cell is that it permits a resistance pattern (for strain gage) to be formed directly on the surface of the ceramic bending beam so that deflection is easily transmitted to the strain gage. The load cell of this structure, therefore, provides improved linearity, creep resistance, hysteresis, and repeatability, which are important characteristics for load cells; it permits measurement with a high accuracy.
Unfortunately, a ceramic material has an inherent disadvantage of having a rough surface. An ordinary ceramic material as baked (without grinding) has a surface roughness of 3.0.mu. R.sub.z. A special ceramic material made of alumina (of purity close to 99.9%) has a surface roughness of 1.0.mu. R.sub.z and 2.0.mu. R.sub.z before and after grinding, respectively. If this rough surface of a ceramic material is provided with a pattern of thin-film resistance (approximately 0.05 .mu.m), it will cause pinholes and uneven thickness distribution in the thin-film resistance. This will present the following problems.
The pattern of thin-film resistance is easily broken by concentrated stress. PA1 The pattern of thin-film resistance does not acquire a desired resistance but greatly varies in resistance. PA1 Resistance varies from one point to another in the single pattern of thin-film resistance; this causes shift errors.
The rough surface of a ceramic material cannot be completely eliminated by lapping or polishing. Lapping or polishing removes surface irregularities; but there still remain interstice between ceramic particles and minute pits due to the loss of ceramic particles caused by grinding. Therefore, the surface roughness that can be attained is about 1.5.mu. R.sub.z, or 0.3.mu. R.sub.z at the best. Moreover, the grinding and lapping of ceramic materials are expensive because they are extremely hard.
The problem of pinhole is solved if a thick-film resistance is formed on the ceramic surface, with the peaks and valleys in the rough surface completely covered. Unfortunately, the resulting thick-film resistance is inferior in follow-up performance to the foil gage. Moreover, the thin-film resistance takes a long time to form, and this leads to a high production cost. Another disadvantage of the strain gage of thin-film resistance is that it has an upper limit in resistance and consumes a large amount of electric power. Therefore, it is not suitable for use in a battery-driven balance.
Incidentally, the surface roughness (R.sub.z) is defined by the difference in height and depth between the third highest peak and the third deepest valley that occur in a cross section of prescribed length.
In the meantime, the load cell of ceramic material is inferior to the conventional one of duralmin (A2024) as discussed in the following.
The load cell of alumina ceramic has a Young's modulus (E.sub.1) of 37,000 kg/mm.sup.2 and a flexural strength of 33 kg/mm.sup.2, whereas the load cell of duralmin has a Young's modulus (E.sub.2) of 7,350 kg/mm.sup.2 and a flexural strength of 33 kg/mm.sup.2.
When a compression-tension strain gage is connected to a Wheatstone bridge, the output (V.sub.OUT) of the bridge is defined by the following equation. EQU V.sub.OUT =V.sub.IN .multidot.K .sigma./E (1) EQU .sigma.=31 w/2 bh.sup.2 (approximate) (2)
where V.sub.IN denotes a voltage applied, K denotes a gage factor, E denotes a Young's modulus, 1 denotes a distance in the horizontal direction between two load-sensing members, b denotes the width of a load-sensing member, and h denotes the thickness of a load-sensing member.
Assuming that the load cell of alumina ceramic and the load cell of duralmin are the same in every respect except Young's modulus (both have usually a gage factor of about 2.0 and both are equal in flexural strength), the ratio of the output V.sub.OUT1 of the ceramic load cell to the output V.sub.OUT2 of the duralmin load cell is equal to the ratio of the Young's modulus E.sub.1 of the ceramic load cell to the Young's modulus E.sub.2 of the duralmin load cell, as indicated by the following equation. EQU V.sub.OUT1 /V.sub.OUT2 =E.sub.2 /E.sub.1 =7350/37000.apprxeq.1/5
In other words, the output V.sub.OUT1 of the ceramic load cell is one-fifth the output V.sub.OUT2 of the duralmin load cell. The low output leads to a considerably decreased accuracy of reading after A/D conversion. Assuming that the output is 1 mV for the input of 1 V (in fact, the output of the duralmin load cell is usually 1-2 mV for the input of 1 V), the following equation is obtained. EQU V.sub.OUT /V.sub.IN =K .sigma./E=10.sup.-3 EQU .sigma.=e/K.times.10.sup.-3
If the load cell of alumina ceramic is to be used under the same condition as above (i.e., 1 mV output for 1 V input), EQU .sigma.=37000/2.0.times.10.sup.-3 =18.5 kg/mm.sup.2
This means that the load cell of alumina ceramic receives an extremely high stress and has a safety factor lower than 2-fold. Under such a situation, the load cell will be broken by a very small impact and will not withstand bending moment and torsional moment except a vertical downward force.
In fact, the stress is higher than this (by about 20%) in the case where output is 1 mV for the input of 1 V. This is because a resistor for temperature compensation is inserted into the input side of the Wheatstone bridge. Therefore, the load cell will give the same safety factor and output as before if it has a gage factor close to 10.
It is empirically known that the load-sensing member of a duralmin load cell can be made as thin as about 0.5 mm, whereas in the case of ceramic load cell, the lower limit of the thickness of the load-sensing member is about 1 mm. It follows, therefore, that the rated load for the ceramic load cell is four times that of the duralmin load cell, at the lowest. In other words, the ceramic load cell cannot measure the weight lower than that rated load.
Despite the advantages of ceramics (as mentioned above), any load cell provided with a load-sensing member of ceramics has not yet been put to practical use.